Method of preventing carbonyl sulfide decomposition



United States Patent METHOD OF PREVENTING CARBONYL SULFIDE DECOMPOSITION Robert W. Foreman, Cleveland, Ohio, assignor to The StantllIard Oil Company, Cleveland, Ohio, a corporation of 0 io No Drawing. Application December 23, 1952, 0

Serial No. 327,701

3 Claims. (Cl. 260-68315) The present invention relates to an improvement in the processing of hydrocarbons in metal equipment. More vention of corrosion in the processing of hydrocarbons containing carbonyl sulfide in metallic apparatus.

There are a number of processes being practiced in the petroleum industry which involve the heating of hydrocarbons, either gaseous or liquid, in metallic apparatus. In some of these processes there has been experienced a considerable amount of corrosion in the metallic apparatus and in others little or no corrosion has been observed. The cause of the corrosion is not always readily ascertainable. Sulfur compounds present in the hydrocarbons as impurities are often blamed, and one of the corrosive sulfur compounds is carbonyl sulfide. In those instances where corrosion of the apparatus has been experienced the cause of the corrosion has not always been found and the difliculties of corrosion have been solved only by the replacement of the metallic apparatus with non-corrosive non-metallic apparatus. It is usually desirable, however, to employ metallic apparatus in many processes whenever possible.

It is an object of this invention to provide a process for preventing the corrosion of metallic apparatus by hydrocarbons containing carbonyl sulfide.

It is still another object of the invention to provide a process for preventing corrosion in metallic apparatus that is employed for the heating of carbonyl sulfide-con taining hydrocarbons.

It is still a further object of the invention to provide a process which substantially prevents the corrosion of metallic equipment employed in the catalytic polymerization of olefins. Other objects will become apparent as the description of the invention proceeds.

Most hydrocarbons contain a small amount of water, and most metallic apparatus contains a surface metal oxide. According to this invention it has been found that the corrosion experienced when hydrocarbons containing carbonyl sulfide and water are heated in metallic apparatus containing a metal oxide is caused by the decomposition of the carbonyl sulfide which is either inherently present or else formed during the processing of the hydrocarbons.

The decomposition of carbonyl sulfide is accelerated by heat and proceeds according to two equations:

The products of the decomposition, particularly sulfur, are known to be corrosive to metal and it Was found essential that the decomposition be prevented from taking place.

The decomposition of carbonyl sulfide according to Equation I when hydrocarbons containing carbonyl sulfide are heated is not surprising in view of the fact that aqueous caustic solutions and/or water are often employed for treating and washing hydrocarbons prior to the processing thereof. The amount of water that remains in the hydrocarbons after contact with aqueous liquids is normally very small, i. e., it usually does not exceed the solubility of water in the hydrocarbon which is generally expressed in parts per million, but this amount is sufficient to hydrolyze a significant quantity of carbonyl sulfide.

The occurrence of Reaction II was likewise not surprising when carbonyl sulfide-containing hydrocarbons are heated in metal equipment because the reaction is reported to be catalyzed by metal oxides, such as iron oxides, which are almost invariably present, even if only in minute quantities, in any type of metallic equipment.

The process of this invention is based on the discovery that both Reactions I and II as outlined above can be prevented by removing substantially all of the water from the carbonyl sulfide-containing hydrocarbons that are to he treated in metallic equipment containing metal oxides. This finding is contrary to all previously known theory or facts which indicated that the presence of water was not essential for the occurrence of Reaction II.

The removal or elimination of water from the hydrocarbons in the practice of this invention can be accomplished by any known means prior to the introduction of the hydrocarbons into metallic apparatus containing metal oxides.

A specific embodiment of this invention may be utilized in the recovery of liquified petroleum gas as a by-product from the catalytic polymerization of olefins. In a typical catalytic polymerization process, an olefinic charging stock is washed with an aqueous caustic solution and I water in order to remove undesirable components. The

washed olefins are then contacted with a polymerization catalyst to produce polymer gasoline. The unpolymerized hydrocarbons resulting from the polymerization step are separated from the polymer gasoline and are then passed to a de-ethanizer wherein C2 hydrocarbons are removed and the residual hydrocarbons, being maintained under pressure, are recovered as liquified petroleum gas. In the commercial operation of a process such as that just described a significant amount of corrosion has often been experienced within the de-ethanizer. It has been found that this corrosion is caused by the decomposition of carbonyl sulfide according to Reaction II and that the decomposition of the carbonyl sulfide can be prevented by removing substantially all of the water from the hydrocarbons prior to their introduction into the de-ethanizer. Thus, the processing stream may be dried at any stage in the catalytic polymerization process intermediate the washing of the olefinic charging stock and the de-ethanization of the unpolymerized hydrocarbons.

Further embodiments of this invention are applicable to other processes in the normal operation of which hydrocarbons containing carbonyl sulfide and water are treated in metallic apparatus containing metal oxides such as alkylation, cracking, distillation, reforming and like processes.

In order to illustrate the improved process of this invention and its accompanying advantages, the following examples are presented.

Example 1 The thermal decomposition of carbonyl sulfide contained in a hydrocarbon was studied using n-heptane. Several seventy-five gram samples of n-heptane containing carbonyl sulfide in difl'erent concentrations were prepared. Water, to the saturation point, and iron oxide in the form of a heavily rusted nail were added to one of the samples andomitted each in turn from others of the samples. The prepared samples were then each placed in a clean dry stainless steel bomb. The bomb was sealed-and piacediman'en"bani which w s a temperature of approximately 275 F. The bomb was allowed to remain inthe hot oilhath for 21 20, hour period and vyas'then removed The top or ,jthehorrih was removed andjthe rusty nail (whenpres ent) withdrawn. A polished copper strip was theninser'ted infthe bomb following the procedure of ASThl test number 4835,'OT, 0.5.ml. of H20 of water was added, andithehomb was sealed and heated for 3 hours at about l22?.l The corrosion of'the copper strips was then rated. The following table presents a summary of the experimental conditions and corrosion test results:

TABLE 1 Varied Conditions 5 I pnu Experiment 38. p.111. n a "D Number OOS' H l. Iron W ater Air ing (STM Oxide 48-35 Ol) ll 0 present... saturated. ptqseutnl 0'1 io .l i-z JifLuui S4) excluded 8- v l l present! 04 I i l 2 1 Parts per million of carbonyl sulfide by weight 2 Air was present in the samples indicated by vii ol its uona'cinor'zrl from the hcadspaco over the heptano. In the air cxclu. i1 experiment, the hcptane was doacratod with nitrogen and the licadspace was completely Example 2 Au olefinic charging stock comprising predominantly a mixture oi propene and butene was washed with an aqueous caustic solution and then with water. The washed olefins were then contacted with a phosphoric acid type polymerization catalyst at an elevated temperature and pressure to form polymer gasoline. The liquid unpolymerized hydrocarbons evolving from the polymerization were separated from the polymer gasoline, dried by contact with anhydrous calcium sulfate and then passed to a de-ethanizer made of iron wherein C2 hydrocarbons were removed by distillation. There was almost a complete absence of corrosion within the de-ethanizer. A repetition of the process in which. the step of drying the hydrocarbons was omitted resulted in a significant amount of corrosion within the de-eihanizer.

The removal of water from the hydrocarbons as practiced according to this invention may beaccompli'shed by any known means. It is preferred to pass the hydrocarbons over a desiccant that is capable of either adsorbing the water or reacting chemically with the water. As examples of such desiccants that can be used there is mentioned calcium carbide, anhydrous calcium sulfate and anhydrous calcium chloride. Instead of employing heated to till a desiccant, however, other means such as refrigeration ofihe'liydro'carb'oiis"accompanied by freezing of the water can be used, although such means are not as economically feasible. It is also possible to remove the water from the hydrocarbons by passing substantially dry air through the hydrocarbons. Other means of drying the hydrocarbons will. be apparent to those skilled in the art.

The improvement of this invention is applicable to any known process for the-treatment of hydrocarbons containing carbonyl sulfide and-Water in which thehydrocarbons are to be contacted with metallic apparatus containing metal oxides. The improvement in reducing or preventing corrosion is most noticeable in those processes wherein the hydrocarbons areheated in contact with the apparatus since the decomposition of carbonyl sulfide is accelerated by heating. However, improvement is also noticeablein processes which involve contact between the hydrocarbon and metal without heating.

The invention-is applicable. to processes iii-which the hydrocarbons are, contacted in either the liquid or gaseous phase with metallic apparatus.

ltis intended to cover all changes and modifications inthe disclosure, of this invention which do not constitute departure from the spirit and scope of the appended claims.

I claim:

'1. In the recovery of liquified petroleum gas by deethanization of the unpolymerized hydrocarbons containing water and carbonyl sulfide resulting from the catalytic polymerization ol olefins, the improvement which comprises drying the unpolymerized hydrocarbons prior to the de-ethanization ot said hydrocarbons.

2. Aprocess which comprises washing an olefinic feed stock containing carbonyl sulfide with an aqueous liquid, contactingthe oiefinic feed with a polymerization catalyst, separating the resultant polymer gasoline and the unpolymerized' hydrocarbons, drying the unpolymerized hydrocarbons to remove substantially all water contained therein, and de-ethanizing the dry unpolymerized hydrocarbons torecover liquified petroleum gas.

3. The process for the prevention of corrosion in metal apparatus containing metal oxides which in normal use is employed to de-ethanize a hydrocarbon stock containing carbonyl sulfide and water in amounts that render the hydrocarbon stock corrosive in the presence of metal oxides, comprising removing substantially all of the water from the hydrocarbon stock prior to introduction of the hydrocarbons containing the unremoved carbonyl sulfide into said metallic oxide-containing metallic apparatus to prevent the corrosion thereof.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN THE RECOVERY OF LIQUIFIED PETROLEUM GAS BY DEETHANIZATION OF THE UNPOLYMERIZED HYDROCARABONS CONTAINING WATER AND CARBONYL SULFIDE RESULTING FROM THE CATALYST POLYMERIZATION OF OLEFINS, THE IMPROVEMENT WHICH COMPRISES DRYING THE UNPOLMERIZED HYDROCARBONS PRIOR TO THE DE-ETHANIZATION OF SAID HYDROCARBONS. 